Regulatory sequences that influence the expression of Ig genes in late stage B cells will be investigated in this application. Based on observations that some IgH enhancer sequences, notably E-mu, can be removed from mature B cells without negatively impacting IgH expression, the determinants that enhance Ig expression in these late stage B cells needs definition. In preliminary experiments, the investigator has showed that deletion of a 3' enhancer element (3'aE) leads to loss of IgG2a expression. Further examination and definition of the control elements regulating IgH gene expression will be evaluated by site-specific deletion of the putative elements in late stage B cells and embryonic stem (ES) cells. In addition to characterizing cis-acting DNA elements in Ig gene expression, the plan is also to prepare a deletion in a gene encoding the Oct-2 transcription factor to determine if the use of this factor is or is not dispensable in late stage B cells.